Power systems for engines, factories, and power plants produce emissions that contain a variety of regulated exhaust constituents. These regulated exhaust constituents may include, for example, particulate matter (e.g., soot), nitrogen oxides (NOx), and sulfur compounds. Due to heightened environmental concerns, engine exhaust emission standards have become increasingly stringent. In order to comply with emission standards, engine manufacturers have developed and implemented a variety of exhaust after-treatment components to reduce pollutants in exhaust gas prior to the exhaust gas being released into the atmosphere. The exhaust after-treatment components may include, for example, a diesel particulate filter (DPF), one or more selective catalytic reduction (SCR) devices, a diesel oxidation catalyst, a heat source for regeneration of the DPF, an exhaust gas recirculation system (EGR), a muffler, as well as other devices.
For example, an SCR system is a means of converting nitrogen oxides, NOx, with the aid of a catalyst, into diatomic nitrogen, N2, and water, H2O. A reductant, typically anhydrous ammonia, aqueous ammonia, or urea, may be added to an exhaust stream between the DPF and SCR system to allow the reductant to be adsorbed onto the catalyst of the SCR system. Carbon dioxide, CO2, is a reaction product when urea is used as the reductant. Gaseous reductants or fluid reductants may be injected into the exhaust stream. When a fluid reductant is used, such a fluid reductant is known as diesel emission fluid, or DEF. The use of DEF has become popular because of its fluid form, which is easy to store and handle, and it has been found that the use of DEF reduces the need to rely upon EGR to meet modern emission requirements.
SCR systems typically include a DEF reservoir and a DEF injector coupled to the reservoir and positioned upstream of the SCR system. The DEF injector injects DEF into a decomposition space or mixing tube through which an exhaust gas stream flows. Upon injection into the exhaust gas stream, the injected DEF spray is heated by the exhaust gas stream to trigger the decomposition of DEF into ammonia. As the DEF and exhaust gas mixture flows through the decomposition tube, the DEF further mixes with the exhaust gas before entering the SCR system. Ideally, the DEF is sufficiently decomposed and mixed with the exhaust gas prior to entering the SCR system to provide an adequately uniform distribution of ammonia at the inlet face of the SCR system.
In such systems, the tip for the DEF injector can be very sensitive to high temperatures. In particular, it has been found that injector life can be significantly shortened when exposed to high temperatures (generally temperatures exceeding around 120 deg. C.) for prolonged periods of time. Accordingly, DEF injector tips may be provided with a cooling system that is arranged to flow cooling fluid from a coolant source, such as an engine radiator, to and around the injector tip during normal operation. Such a system has been found to effectively prevent the injector tip from prolonged exposure to undesirably high temperatures during regular operations as well as during normal shutdown operations which include a “cool down” period and other protocols allowing the engine, and associated pumps, to continue to run at low-stress conditions for a period of time following heavy use. However, when a shutdown occurs where such protocols cannot be followed (for whatever reason), particularly following strenuous use (known as a “hot” shutdown), the injector tip can be subjected to temperatures 30-40 C above the desired limit thus potentially shortening the life of the injector tip.
Some prior art methods of cooling DEF injector tips in such circumstances have included flowing DEF fluid through the injector tip following shutdown and/or adding cooling fins to the injector to aid in air cooling of the injector tip. For example, US Publication No. 2013/0291523 A1 filed on May 2, 2012 and assigned to Caterpillar, Inc. discloses an apparatus for cooling a DEF injector tip through the use of a heat shield and air cooling provided by a supplementary cooling duct. However, these methods and apparati have been somewhat disadvantageous for multiple reasons, including, but not limited to, deposit formation on the injector tips, waste of DEF fluid, and insufficient cooling.